A liquid-crystal display device generally comprises a liquid-crystal cell and a polarizing plate. The polarizing plate has a protective film and a polarizing film, and this is obtained, for example, by dyeing a polarizing film formed of a polyvinyl alcohol film with iodine, stretching it and then laminating a protective film on both faces thereof. In general, a transmission liquid-crystal display device may comprise such a polarizing plate fitted to both sides of the liquid-crystal cell thereof, and may further has one or more optical compensatory film provided therein. A reflection liquid-crystal display device may generally comprise a reflector, a liquid-crystal cell, one or more optical compensatory films and a polarizing plate disposed in that order. The liquid-crystal cell comprises liquid-crystal molecules, two substrates for sealing them in, and an electrode layer for imparting voltage to the liquid-crystal molecules. Depending on the alignment state of the liquid-crystal molecules therein, the liquid-crystal cell acts for ON/OFF display, for which proposed are various display modes of TN (twisted nematic), ISP (in-plane switching), OCB (optically-compensatory bent), VA (vertically-aligned) and ECB (electrically-controlled birefringence) modes that are applicable to both transmission and reflection devices.
Of such LCDs, nematic liquid-crystal molecules having positive dielectric anisotropy are used for those that are required to have high display quality, for which essentially used are 90-degree twisted nematic liquid-crystal display devices (hereinafter referred to as TN mode) driven by thin-film transistors. However, the TN mode has such viewing-angle characteristics that it has excellent display characteristics when seen in the front direction thereof but its display characteristics worsen when seen in the oblique direction in that its contrast lowers and it causes grayscale inversion which means brightness inversion in grayscale state, and improving it is greatly desired.
To solve the problem, an in-plane switching (IPS) mode liquid-crystal display device in which a horizontal electric field is applied to the liquid crystal, and a vertically-aligned (VA) mode device in which liquid crystals having negative dielectric anisotropy are vertically aligned and the alignment is divided by projection or slit electrodes formed inside the panel have been proposed and put into practical use. These days the panels are not limited to monitor applications but are developed for TV applications, and with that, the screen brightness of the panels is greatly increasing. Accordingly, minor light leakage in the diagonal oblique incident direction in a black state, which, however, has not heretofore been considered problematic in those display modes, has become considered as a problem for the reasons of display quality depression.
One method for improving the color tone and the viewing angle in a black state is now investigated for IPS mode devices, which comprises disposing an optically-compensatory material having birefringence characteristics between a liquid-crystal layer and a polarizing plate. For example, it is disclosed that, when birefringent media each having an action of compensating the retardation change in a tilted liquid-crystal layer are disposed between a substrate and a polarizing plate in such a manner that their optical axes are perpendicular to each other, then the discoloration in white display or in intermediate tone display in the oblique direction can be reduced (see JPA No. 9-80424). On the other hand, proposed is a method of using an optical compensatory film that comprises a styrene-based polymer or discotic liquid-crystal compound having a negative intrinsic birefringence (see JPA NO. 10-54982, 11-202323, 9-292522). However, many of the proposed systems are for canceling the birefringence anisotropy of the liquid crystal in a liquid-crystal cell to thereby improve the viewing angle characteristics of display devices, and are therefore still problematic in that they could not solve the problem of light leakage to be caused by the shifting of the cross angle of the polarization axes from a right angle when the polarizing plates set at right angles are seen in the oblique direction. Even in a system in which the light leakage could be compensated, it is still extremely difficult to completely optically compensate the liquid-crystal cell with no problem. This is because even when light leakage could be completely compensated at a certain wavelength in the system, light leakage at any other wavelength could not be always compensated therein. For example, even when light leakage at a wavelength of green having a largest visibility could be compensated therein, the system would be still problematic in that it could not prevent light leakage at a smaller wavelength of blue or at a larger wavelength of red. To solve this problem, lamination of two biaxial films is proposed in Jpn. J. Appl. Phys., 41, (2002) 4553. However, two biaxial films are used therein, and therefore the method is still problematic in that the axial shifting of the biaxial films may readily occur and the image plane may be often uneven. In addition, the light leakage in a black state may be caused by the in-plane retardation Re and the retardation Rth in the thickness direction of the triacetylcellulose film heretofore used as the polarizing plate-protective film between a liquid-crystal cell and a polarizer.